Xiaomi R3P on snapshot: Really low wireless speed

I have some problems with wireless speed on Xiaomi Router Pro 3 on ±latest snapshot.

Model Xiaomi Mi Router 3 Pro
Architecture MediaTek MT7621 ver:1 eco:3
Firmware Version OpenWrt SNAPSHOT r15676-c3959cd54f / LuCI Master git-21.029.67968-af1f961
Kernel Version 5.4.94

netperf (and real world tests) show 2-3 Mbytes per second on 5GHz, when it is clearly capable of 100 Mb speeds. Connecting via cable shows, that netperf can produce these speeds.

Testing with other network devices shows, that the problem is indeed in the router.

I reset several times, trying multiple AP modes and bandwidths — nothing yielded speed improvements.
Also I tried mainline 19.07.

Is there probably some blob I need to flash to get the proper speeds?